The invention relates to a system and method for controlling the flow and pressure of fluid, and, more particularly, to such a system and method according to which a choke is provided that automatically moves to control the flow of fluid in a system and to maintain a desired back pressure on the fluid no matter what flow conditions exist or occur within the system.
There are many applications in which there is a need to control the back pressure of a fluid flowing in a system. For example, in the drilling of oil wells it is customary to suspend a drill pipe in the well bore with a bit on the lower end thereof and, as the bit is rotated, to circulate a drilling fluid, such as a drilling mud, down through the interior of the drill string, out through the bit, and up the annulus of the well bore to the surface. This fluid circulation is maintained for the purpose of removing cuttings from the well bore, for cooling the bit, and for maintaining hydrostatic pressure in the well bore to control formation gases and prevent blowouts, and the like. In those cases where the weight of the drilling mud is not sufficient to contain the bottom hole pressure in the well, it becomes necessary to apply additional back pressure on the drilling mud at the surface to compensate for the lack of hydrostatic head and thereby keep the well under control. Thus, in some instances, a back pressure control device is mounted in the return flow line for the drilling fluid.
Back pressure control devices are also necessary for controlling "kicks" in the system caused by the intrusion of salt water or formation gases into the drilling fluid which may lead to a blowout condition. In these situations, sufficient additional back pressure must be imposed on the drilling fluid such that the formation fluid is contained and the well controlled until heavier fluid or mud can be circulated down the drill string and up the annulus to kill the well. It is also desirable to avoid the creation of excessive back pressures which could cause drill string to stick, or cause damage to the formation, the well casing, or the well head equipment.
However, maintenance of an optimum back pressure on the drilling fluid is complicated by variations in certain characteristics of the drilling fluid as it passes through the back pressure control device. For example, the density of the fluid can be altered by the introduction of debris or formation gases, and/or the temperature and volume of the fluid entering the control device can change. Therefore, the desired back pressure will not be achieved until appropriate changes have been made in the throttling of the drilling fluid in response to these changed conditions. Conventional devices generally require manual control of and adjustments to the choking device orifice to maintain the desired back pressure. However, manual control of the throttling device involves a lag time and generally is inexact.
U.S. Pat. No. 4,355,784, assigned to the assignee of the present application, discloses an apparatus and method for controlling back pressure of drilling fluid in the above environment which addresses the problems set forth above. According to this arrangement, a balanced choke device moves in a housing to control the flow and the back pressure of the drilling fluid. One end of the choke device is exposed to the pressure of the drilling fluid and its other end is exposed to the pressure of a control fluid. However, in this design, when the choke device moves from its closed position to an open position in response to either an increase in the well fluid pressure or a decrease in the control fluid pressure, it often moves too quickly and too far, thus often overshooting its normal operating position. This can cause pressure spikes and/or can allow a large volume of gas near the surface of the well to vent out of the device, both of which are undesirable.
Therefore, what is needed is a method and system for controlling back pressure of a flowing fluid which incorporates all the advantages of the system and method disclosed in the above patent, yet reduces any pressure spikes and gas venting when the choke moves from its closed position to an open position.